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This graph shows annual sunspot counts from the year 1600 to 2000. Note the extreme lack of sunspots during the Maunder Minimum, and the smaller dip in sunspot counts during the Dalton Minimum.
Click on image for full size Image courtesy NASA (modified by Windows to the Universe staff).

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History of Sunspot Observations

Surprisingly, humans have observed sunspots for a very long time, so historical
sunspot observations provide us with some of our best long-duration records
of solar activity. Large
sunspots can sometimes be seen with the naked eye,
especially when the Sun is viewed through fog near the horizon at sunrise or
sunset. (WARNING: Never look directly at the Sun! Even a brief glance can damage
your eyes!) The first written record of sunspots was made by Chinese astronomers
around 800 B.C. Court astrologers in ancient China and Korea, who believed
sunspots foretold important events, kept sporadic records of sunspots for hundred
of years. An English monk named John of Worcester made the first drawing of
sunspots in December 1128.

Soon after the invention of the telescope, several astronomers made the first
telescopic observations of sunspots. Galileo
Galilei in Italy, Johann Goldsmid
(a.k.a. Fabricius) in Holland, Thomas Harriot in England, and Christoph Scheiner
in Germany all viewed sunspots through telescopes between 1610 and 1613. Astronomers
of that era weren't quite sure what to make of these spots on the Sun; some
thought they were silhouettes of previously undiscovered planets
transiting the Sun as they orbited closer than Mercury,
while others believed they were dark clouds in the Sun's
atmosphere. The progression of sunspots across the face of the Sun allowed
early seventeenth century astronomers to make the first estimates of the Sun's
rotation period (about 27 days).

Very few sunspots were seen from about 1645 to 1715, a period referred to
as the Maunder Minimum after the English astronomer Edward W. Maunder who studied
this unusual time of solar inactivity. This period corresponds to the middle
of a series of exceptionally cold winters throughout Europe known as the Little
Ice Age. Scientists still debate whether decreased solar activity
helped cause the Little Ice Age, or if the cold snap coincidentally occurred
around the same time as the Maunder Minimum. Several other less-extreme periods
of
decreased sunspot activity have been noted: the Spörer Minimum (1420 to
1570), named after the German astronomer Gustav Spörer; the Dalton
Minimum of 1790 to 1820; the Wolf Minimum of 1280 to 1340; and the Oort
Minimum of 1010 to 1050. Indirect evidence from elemental isotopes seemingly
indicates
that
there have
been 18 such periods of lessened sunspots over the last 8,000 years and that
the Sun may spend as much as a quarter of its time in such sunspot minima periods.
The Spörer, Wolf, and Oort Minima were all discovered via such isotope
analyses, since they occurred before the era of regular, reliable sunspot observations.
In contrast to these periods of sunspot minima, sunspot counts have
been higher than usual since around 1900, which has led some scientists to
call
this time the Modern Maximum. Likewise a period called the Medieval Maximum,
which lasted from 1100 to 1250, apparently had higher levels of sunspots and
associated solar activity, and intriguingly coincides (at least partially)
with a period of warmer climates on
Earth called the Medieval
Warm Period.

In 1843 an amateur German astronomer named Samuel Heinrich Schwabe, using
17 years of his personal sunspot observations, discovered the rise and fall
of yearly sunspot counts we now call the sunspot
cycle. He initially estimated the cycle's length at 10 years. Two French
physicists, Louis Fizeau and Léon Foucault, took the first photo of
the Sun and sunspots in April 1845. Around 1852 four astronomers noted, roughly
simultaneously, that the period of the sunspot cycle was identical to the period
of variation of geomagnetic activity at Earth, giving birth to the field of
study of Sun-Earth connections we now call "space
weather".

Around 1858 the Englishman Richard C. Carrington and Gustav Spörer independently
made two important discoveries: the solar latitude at which sunspots appear
gradually decreases from about 40° to 5° throughout the course of a
sunspot cycle (now often called Spörer's Law), and sunspots at different
latitudes move around the Sun at different rates. The latter fact led them
to conclude that the Sun does not rotate as a solid sphere, but rather has
different
rates of rotation at different latitudes (about 30% slower near the
poles than near the equator) characteristic of a gaseous body. In 1868 the
Swiss astronomer Rudolf Wolf was trying to compare historical sunspot counts
by many different astronomers using various instruments and observing techniques.
He devised a formula, which is still
in use today, that combined data about counts of individual spots,
counts of sunspot groups, and a correction factor for each observer. The result
of
his calculation for any given period is called the "Wolf sunspot
number".

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